The present invention provides a method and an apparatus for extruding an extrudate with a controlled bulk density. In particular, the present invention provides an extruder with a variable restriction element which can be used to control the bulk density of the extrudate as the extrudate is extruded from the extruder, and a process of controlling the bulk density of an extrudate with a variable restriction element.
Extruders are commonly used to extrude thermoplastic materials such as plastics and moisturized feedstuffs formed of farinaceous and proteinaceous materials. The extruder heats and pressurizes the thermoplastic material and forces the material through an extruder die, whereupon the extruded material is cut into a desired shape as it exits the extruder die. For example, in the formation of dry kibbled pet foods a mixture of farinaceous and proteinaceous material is mixed with water, plasticized, and cooked as it is pushed through the extruder under pressure, and is subsequently cut into kibbles as it is extruded through the extruder die.
Maintenance of an extrudate having a constant bulk density is important to maintain consistent product size and package weights in the production of extruded foods. A common method to maintain the bulk density of extruded foodstuffs at constant levels is to adjust the moisture content of the material being extruded, which varies the inherent lubricity of the extrudate. The bulk density of the extrudate may be increased by increasing the moisture content of the material being extruded since higher levels of moisture reduce the energy imparted to the extrudate by the extruder screw, reducing expansion of the extrudate. Inversely, decreasing the moisture content of the material being extruded increases the expansion of the extrudate which decreases the bulk density of the extrudate. Moisture levels in the material can be controlled by adding steam or water to the material prior to extrusion, or injecting steam or water into the material as is passes through the extruder.
Use of the moisture content of the extruded material to control the bulk density of the extrudate subsequent to extrusion introduces variability into the quality of the final food product. Typically, an extrudate is dried in a dryer after being extruded though the die and cut to the desired size. Extrudate containing different levels of moisture will dry to an extent dependent on the amount of moisture in the extrudate when the extrudate is dried for a set period of time in a dryer. Extrudates containing relatively little moisture will tend to be burnt by the drying process, and extrudates containing excessive moisture will not completely dry, leading to a food product susceptible to mold.
In large scale commercial applications several extruders are often serviced by one dryer, and the variations in moisture levels of different extrudates entering the dryer can be large, resulting in great variation in the product exiting the dryer. The ultimate quality of the product may be adversely affected by these variations. For example, a burnt pet food kibble may not be palatable to the pet for which it is intended, and a moist pet food kibble may be unacceptable because of mold spoilage.
A recently published article authored by S. Shonauer and R. Moreira entitled A Variable Restrictive Valve as an Extra Independent Control Variable For Food Extrusion Processes (Food Science and Technology International, Vol. 2, pp. 241-48 (1996)) discloses the use of a restriction valve located in the die of an extruder extending across the die outlet orifice as a method of controlling the bulk density of a food extrudate without changing the moisture content of the extrudate. At relatively closed restriction valve positions the bulk density of the extrudate is low since the extrudate expands significantly as it exits the restricted die orifice under pressure from the extruder screw. At relatively open restriction valve positions the bulk density of the extrudate is high since little expansion occurs as the extrudate exits the die orifice.
The die restriction valve of Shonauer and Moreira, while permitting control of the bulk density of an extruded product, causes variations in the extruded product as the valve is positioned at various levels of restriction. Throttling the die restriction valve changes the die geometry and produces a thinner extrudate. For production of extruded foods having uniform characteristics, use of a die restriction valve is not particularly desirable since the die restriction valve causes variations in the thickness of the extrudate.
Further, Shonauer and Moreira do not suggest that a die restriction valve can be adjusted to continuously control bulk density while the extruder is in operation. In a commercial food extruding process the ability to continuously adjust the bulk density without altering the moisture level of the food in the extruder while the extruder is in operation is desirable to produce a uniform product.